As shown in FIGS. 1 and 2, a prior art cradle B is composed of a support frame A and a base C. The support frame A is fastened at the front end threrof with the base C by means of a flexible means D such that the cradle B can be caused to move up and down in a reciprocating manner. The support frame A is provided at the rear end A1 thereof with a fastening rod E which is in turn provided with a permanent magnet E. The base c is provided at the rear end thereof with an iron core F which is fitted into a coil and is provided on the back thereof with a planar seat F1. The permanent magnet E is coplanar with the iron core F such that the permanent magnet E can be so adjusted as to locate at the same level as the iron core F. When the permanent magnet E is moved up and down, the circuit control coil of the iron core F brings about the repulsive magnetic field to keep the cradle B moving up and down. In other words, the cradle B is kept in the normal operating state only when the permanent magnet E and the iron core F at located at the same level. However, such an equilevel state of the permanent magnet E and the iron core F as described above can be upset easily by a change in the weight of an infant. As shown in FIG. 2, the prior art cradle B is provided with an outer tube E2 which is located over the fastening rod E1 for correcting the level at which the permanent magnet E is located. As the infant grows up rapidly, the level of the permanent magnet E must be adjusted from time to time so as to keep the cradle B operating normally. In addition, the repulsive magnetic field brought about by the magnet E and the iron core F is vulnerable to destruction at such time when the cradle B happens to be near a magnetic object having a magnetic field greater than the magnetic field of the cradle B. Moreover, the prior art cradle B can be caused to fail to operate by an iron block which is accidentally or playfully misplaced between the permanent magnet E and the iron core F.